Sensor Part-III : Draw a Compass use Position Sensor

Hi ... Dear All ..Today I am uploading a superb example to draw Compass using Position Sensor and Accelerometer.

Please check.
I am using Android Studio 1.5.1
Minimun SDK API 19
Target SDK API 23
Please Like us & put your valuable suggestions in comment box.
***** check on physical device
*** No layout xml file we use canvas to draw

A. Java File MainActivity.java

package com.exam.ravi.sensorex2;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;

public class MainActivity extends AppCompatActivity implements SensorEventListener {

    Float drawValue;

    public class CustomDrawableView extends View {
        Paint paint = new Paint();
        public CustomDrawableView(Context context) {
            super(context);
            paint.setColor(Color.RED);
            paint.setStyle(Paint.Style.STROKE);
            paint.setStrokeWidth(4);
            paint.setAntiAlias(true);
        };

        protected void onDraw(Canvas canvas) {
            int width = getWidth();
            int height = getHeight();
            int centerx = width/2;
            int centery = height/2;
            canvas.drawLine(centerx, 0, centerx, height, paint);
            canvas.drawLine(0, centery, width, centery, paint);

            if (drawValue != null)
                canvas.rotate(-drawValue*360/(2*3.14159f), centerx, centery);
            paint.setColor(Color.GREEN);
            canvas.drawLine(centerx, -1000, centerx, +1000, paint);
            canvas.drawLine(-1000, centery, 1000, centery, paint);
            canvas.drawText("N", centerx+5, centery-10, paint);
            canvas.drawText("S", centerx-10, centery+15, paint);
            paint.setColor(Color.RED);
        }
    }

    CustomDrawableView drawableView;
    private SensorManager manager;
    Sensor accelerometer;
    Sensor magnetometer;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        drawableView = new CustomDrawableView(this);
        setContentView(drawableView);    // Register the sensor listeners

        manager = (SensorManager)getSystemService(SENSOR_SERVICE);
        accelerometer = manager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        magnetometer = manager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);
    }

    protected void onResume() {
        super.onResume();
        manager.registerListener(this, accelerometer, SensorManager.SENSOR_DELAY_UI);
        manager.registerListener(this, magnetometer, SensorManager.SENSOR_DELAY_UI);
    }

    protected void onPause() {
        super.onPause();
        manager.unregisterListener(this);
    }

    public void onAccuracyChanged(Sensor sensor, int accuracy) {  }

    float[] myGravity;
    float[] myGeomagnetic;
    public void onSensorChanged(SensorEvent event) {
        if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER)
            myGravity = event.values;
        if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD)
            myGeomagnetic = event.values;
        if (myGravity != null && myGeomagnetic != null) {
            float R[] = new float[9];
            float I[] = new float[9];
            boolean success = SensorManager.getRotationMatrix(R, I, 
                                          myGravity, myGeomagnetic);
            if (success) {
                float orientation[] = new float[3];
                SensorManager.getOrientation(R, orientation);
                drawValue = orientation[0];
            }
        }
        drawableView.invalidate();
    }
}